U.S. patent application number 13/801086 was filed with the patent office on 2013-11-21 for glass block dichroic beamsplitters.
This patent application is currently assigned to SEMROCK, INC.. The applicant listed for this patent is SEMROCK, INC.. Invention is credited to Turan Erdogan, Prashant Prabhat, Ligang Wang.
Application Number | 20130308198 13/801086 |
Document ID | / |
Family ID | 49581098 |
Filed Date | 2013-11-21 |
United States Patent
Application |
20130308198 |
Kind Code |
A1 |
Erdogan; Turan ; et
al. |
November 21, 2013 |
GLASS BLOCK DICHROIC BEAMSPLITTERS
Abstract
A dichroic beamsplitter has a composite prism that has at least
first and second prism elements that are coupled together along
facing surfaces, wherein the respective facing surfaces of the
first and second prism elements are equidistant from each other.
The composite prism has a first flat external surface that lies
within a first plane, a second flat external surface that lies
within a second plane that is perpendicular to the first plane, a
third flat external surface that lies within a third plane that is
parallel to the second plane, and a coated surface internal to the
composite prism and having a multilayer thin-film dichroic
beamsplitter coating, wherein the coated surface lies within a
fourth plane that intersects at least one of the first, second, and
third planes at an angle that is less than about 25 degrees.
Inventors: |
Erdogan; Turan;
(Spencerport, NY) ; Wang; Ligang; (Cupertino,
CA) ; Prabhat; Prashant; (Rochester, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMROCK, INC. |
Rochester |
NY |
US |
|
|
Assignee: |
SEMROCK, INC.
Rochester
NY
|
Family ID: |
49581098 |
Appl. No.: |
13/801086 |
Filed: |
March 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61648242 |
May 17, 2012 |
|
|
|
Current U.S.
Class: |
359/640 |
Current CPC
Class: |
G02B 27/142 20130101;
G02B 27/126 20130101; G02B 27/149 20130101; G02B 27/1006
20130101 |
Class at
Publication: |
359/640 |
International
Class: |
G02B 27/12 20060101
G02B027/12 |
Claims
1. A dichroic beamsplitter comprising: a composite prism that has
at least first and second prism elements that are coupled together
along facing surfaces, wherein the respective facing surfaces of
the first and second prism elements are equidistant from each
other, wherein the composite prism has a first flat external light
incidence surface that lies within a first plane, a second flat
external light incidence and exit surface that lies within a second
plane that is perpendicular to the first plane, a third flat
external light exit surface that lies within a third plane that is
parallel to the second plane, and a coated surface internal to the
composite prism and comprising a multilayer thin-film dichroic
beamsplitter coating, wherein the coated surface lies within a
fourth plane that intersects at least one of the first, second, and
third planes at an angle that is less than about 25 degrees.
2. The dichroic beamsplitter of claim 1 wherein the composite prism
forms a penta prism.
3. The dichroic beamsplitter of claim 1 wherein the composite prism
is formed from three prism elements and includes an air gap between
the first and second prism elements.
4. The dichroic beamsplitter of claim 1 wherein the beamsplitter
coating on the coated surface provides a short wavelength pass
filter.
5. The dichroic beamsplitter of claim 1 wherein the beamsplitter
coating on the coated surface provides a long wavelength pass
filter.
6. The dichroic beamsplitter of claim 1 wherein the facing surfaces
of the first and second prism elements lie in the fourth plane.
7. The dichroic beamsplitter of claim 1 wherein the respective
facing surfaces of the first and second prism elements are in
optical contact with each other.
8. The dichroic beamsplitter of claim 1 wherein the respective
facing surfaces of the first and second prism elements are spaced
apart by an air gap.
9. The dichroic beamsplitter of claim 1 wherein the multilayer thin
film coating is a first multilayer thin film coating and wherein
there is a second multilayer thin film coating on the first flat
external surface of the composite prism.
10. The dichroic beamsplitter of claim 1 wherein the multilayer
thin film coating is a first multilayer thin film coating and
wherein there is a second multilayer thin film coating on the
second flat external surface of the composite prism.
11. The dichroic beamsplitter of claim 1 wherein the at least first
and second prisms are substantially identical in shape.
12. The dichroic beamsplitter of claim 1 wherein the multilayer
thin film coating transmits multiple wavelength bands and reflects
light having wavelengths that lie between the transmitted
wavelength bands.
13. A beamsplitter comprising: a composite prism that has at least
first and second prism elements that are coupled together along
facing surfaces and are either in optical contact with each other
along the facing surfaces or spaced apart along the facing surfaces
by a fixed gap, wherein the composite prism has a first planar
external surface, a second planar external surface that is
perpendicular to the first planar external surface, a third planar
external surface that is parallel to the second planar external
surface, and a fourth planar surface within the composite prism,
formed on one or both of the facing surfaces, and having a
multilayer thin-film dichroic beamsplitter coating that is treated
to reflect and transmit light of different wavelengths, wherein the
fourth planar surface of the composite prism directs light from a
light source of a first wavelength that is incident at a normal to
the first planar surface to be output at a normal to the second
planar surface and directed toward a sample and directs light of a
second wavelength excited from the sample that is incident at a
normal on the second planar surface to be output at a normal from
the third planar surface, and wherein an angle of intersection of
the fourth planar surface with one of the first, second, or third
planar external surfaces is less than 25 degrees.
14. The beamsplitter of claim 13 wherein the first prism is a penta
prism.
15. The beamsplitter of claim 13 wherein the fixed gap is an air
gap.
16. The beamsplitter of claim 13 wherein the composite prism is
formed from a Pechan prism and a wedge prism.
17. A glass cube beamsplitter comprising: a composite prism that
has at least first and second prism elements that are coupled
together in optical contact along facing surfaces, wherein the
composite prism comprises: (i) a first planar external surface
having a first multilayer thin-film dichroic coating, (ii) a second
planar external surface that is perpendicular to the first planar
external surface, (iii) a third planar external surface that is
parallel to the second planar external surface and having a second
multilayer thin-film dichroic coating, and (iv) a fourth planar
surface within the composite prism, formed on one or both of the
facing surfaces, and having a third multilayer thin-film dichroic
coating that is treated to reflect and transmit light of different
wavelengths, wherein the third multilayer thin-film dichroic
coating of the composite prism directs light of a first wavelength
that is incident at a normal to the first planar surface to be
output at a normal to the second planar surface and directs light
of a second wavelength that is incident at a normal on the second
planar surface to be output at a normal from the third planar
surface, and wherein an angle of intersection of the fourth planar
surface with one or more of the first, second, and third planar
external surfaces is less than 25 degrees.
18. The beamsplitter of claim 17 wherein the fourth planar surface
intersects one of the first, second, and third planar external
surfaces along an edge of the beamsplitter.
19. The beamsplitter of claim 17 wherein the first multilayer
thin-film dichroic coating transmits the light of the first
wavelength and wherein the second multilayer thin-film dichroic
coating blocks the light of the first wavelength.
20. The beamsplitter of claim 13 further comprising third and
fourth prisms and wherein the beamsplitter further has fifth and
sixth planar surfaces that reflect light of the first wavelength
that is incident from within the beamsplitter.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/648,242 filed May 17, 2012 in the
names of Turan Erdogan et al., the contents of which are
incorporated fully herein by reference.
FIELD OF THE INVENTION
[0002] This invention generally relates to apparatus and methods
for separation of light paths according to spectral content and
more particularly relates to dichroic beamsplitter prisms.
BACKGROUND OF THE INVENTION
[0003] Optical beamsplitters are used in a number of different
applications. In spectroscopy and other instrumentation systems,
for example, a beamsplitter is used to direct excitation energy of
one wavelength toward a sample along a first optical path and to
direct emitted light that has been excited from the sample to
sensing components along a second optical path, which can be in the
opposite direction from some portion of the first optical path. The
beamsplitter reflects light of at least one wavelength band and
transmits light of another wavelength band or bands.
[0004] The schematic diagram of FIG. 1 shows one exemplary optical
apparatus that uses a beamsplitter. In a fluorescence microscope
10, excitation light of wavelength .lamda.1, directed through a
lens 34 from a light source 12, is first filtered by an excitation
filter 20 and then reflected from a dichroic beamsplitter 22 before
being directed to a sample 14 through an objective lens 24.
Fluorescent molecules in sample 14 absorb this light, and then emit
longer-wavelength fluorescence light of wavelength .lamda.2, some
of which is captured by the objective lens 24 and imaged through
one or more secondary lenses 26 onto a detector 28 (such as a human
eye or charge-coupled device or CCD camera). The fluorescence is
transmitted through the same dichroic beamsplitter 22 as well as
through an emission filter 32 which is required to block all
unwanted background light as well as light from the excitation
light source 12.
[0005] A similar configuration to that shown in FIG. 1 is generally
used for other types of spectroscopy measurement systems in which a
generally shorter-wavelength excitation light source generates
light at longer wavelengths to be spectrally detected, such as
Raman spectroscopy. For reasons of mechanical convenience,
compactness, and stability, it is generally desirable for the
excitation light to be incident on the imaging path at a 90.degree.
angle, such as shown in FIG. 1. This light is incident on the
dichroic beamsplitter at 45.degree..
[0006] In many systems a plate dichroic beamsplitter is used, which
comprises a multilayer thin-film coating applied to one surface of
a thin parallel plate of glass. The back surface of this filter may
be anti-reflection coated to minimize loss of light and extraneous
reflections.
[0007] In practice, a plate dichroic beamsplitter is a workable
solution only when the light between the objective lens 24 and the
secondary lens 26 is highly collimated, as in the FIG. 1 example.
If this light is not collimated, however, the tilted parallel plate
of glass on which the thin-film coating is formed causes increased
spherical aberration as well as appreciable asymmetric aberrations,
such as coma and astigmatism, at the image. Furthermore, the tilted
plate causes a slight lateral shift of the optical axis OA. In some
systems it is not desirable or possible to tolerate a lateral shift
of the beam of image-bearing light, and therefore, of the optical
axis, as caused by the tilted parallel plate.
[0008] To avoid these aberrations in systems where the light is not
collimated, as well as to minimize or eliminate the lateral beam
shift, some systems use cube or cubic dichroic beamsplitters, as
shown in FIG. 2. A cube dichroic beamsplitter 120 is formed as a
type of composite prism from two right-angle component prism
elements 122a and 122b, each joined to the other along the facing
surface that is its hypotenuse, with a multilayer thin-film coating
124 applied to one hypotenuse or the other hypotenuse. This
arrangement embeds dichroic coating 124 within the glass substrate.
Optical contact between the component prism elements can be
effected in a number of ways familiar to those skilled in the
optical arts, such as using an index-matching cement, or employing
what is known as direct optical contact (glass-to-glass bonding by
weak van der Waal's forces), or with strong, chemically activated
molecular glass-to-glass bonding. In some cases, there can also be
a fixed gap such as an air gap maintained between the two component
prisms. Because light only enters and exits any glass surface at or
near normal incidence (0.degree.) with these prisms, the cube
beamsplitter approach solves the optical aberration and beam shift
problems described earlier.
[0009] One notable drawback of the cube approach, however, relates
to high angles of light incidence on the embedded multilayer
thin-film coating 124 that lies within the prism. It is well-known
that filter response for thin film filters changes with angle, so
that multilayer thin film coatings tend to degrade in performance
as the angle of incidence increases.
[0010] With the plate beamsplitter, as in FIG. 1, light is incident
on the dichroic surface at 45.degree. in air. However, due to
Snell's law of refraction, the light bends upon entering the glass
substrate, so that its incident angle, relative to the thin film
layers coated on the glass, is refracted to about 28.degree.
(assuming an index of refraction near 1.5 for the glass substrate
of beamsplitter 22). This is a suitable angle for reasonable
dichroic coatings performance and the plate beamsplitter 22 can
provide effective separation of light for many applications with
incident light in this range.
[0011] However, the case is different with the cube beamsplitter.
Light traveling within the cube substrate does not refract as it
nears the multilayer thin-film coating and is incident on the
thin-film coating at a much higher angle of incidence than it is in
the case of the plate beamsplitter. It is much more difficult to
design and fabricate a dichroic beamsplitter coating with a steep,
well-defined edge transition between reflection and transmission
for light incident at 45.degree.. At higher incidence angles,
polarization differences compromise beamsplitter performance.
P-polarized light experiences much lower reflection than
s-polarized light, and the wavelength location of a filter edge
tends to be very different for s- and p-polarized light. This
behavior, termed "polarization splitting", tends to broaden
transition edges of the filters. As a result, the spectral
performance of the cube dichroic beamsplitter that has an embedded
coating can be disappointing, resulting in poorer overall system
efficiency and, in many cases, resulting in lower signal
sensitivity.
[0012] Conventional cube beamsplitter designs that use embedded
multilayer thin-film coatings are hampered by poor performance at
high incidence angles and are unable to benefit from the advantages
dichroic beamsplitter thin-film coatings have at lower angles of
incidence. For example, fewer thin film layers are needed for a
given amount of reflectivity or edge steepness at lower angles of
incidence. This has advantages of reduced cost and improved edge
steepness over multilayer coatings designed for higher incidence
angles. As yet another consideration, a coating with fewer layers
generally also exhibits lower group delay dispersion (all other
performance parameters being equal), with significantly improved
performance for beamsplitters that reflect femtosecond laser
pulses.
[0013] Still another consideration relates to the demands of the
optical system itself. Various types of composite prisms with
embedded dichroic coatings or coatings applied to one or more
surfaces have been designed for color splitting or combining, such
as in camera and projection apparatus, for example. None of these
conventional solutions, however, is well-suited for use in a
spectroscopy measurement system. In the spectroscopy apparatus,
input light at one wavelength is reflected toward a sample along an
optical imaging path, while light of a different, typically longer
wavelength is transmitted through the beamsplitter along the same
optical path in the opposite direction, entering and exiting the
glass block at or near normal incidence.
[0014] In summary, although the cube beamsplitter has clear
advantages that relate to mounting, light handling, and durability,
the poor relative spectral performance of these devices makes them
less desirable than plate beamsplitters for light separation in
many applications.
[0015] Therefore, there is a need for a dichroic beamsplitter cube
that supports orthogonal input and output light paths, that takes
advantage of the low aberration and beam shift of a glass cube or
prism when contrasted with a plate dichroic in imperfectly
collimated light, and that has improved spectral performance over
conventional cube beamsplitter designs.
SUMMARY OF THE INVENTION
[0016] It is an object of the present invention to advance the art
of spectral separation of light. Dichroic beamsplitter solutions
presented herein reduce or eliminate aberrations and beam shift
associated with conventional plate dichroics when used in
imperfectly collimated light. Embodiments of the present invention
provide improved spectral performance over conventional cube or
prism beamsplitter designs using arrangements that reduce the angle
of incidence to internal thin-film surfaces to well below
45.degree.. Embodiments of the present invention employ a glass
block assembly that enables an incident beam of light at one range
of wavelengths to enter the glass block assembly at, or near,
normal incidence and to be reflected at a 90.degree. angle into the
main optical imaging path, while simultaneously enabling light at
another range of wavelengths to enter and exit the glass block
assembly at or near normal incidence at both interfaces and along
the same optical imaging path, such that both beams of light are
incident on an embedded multilayer thin-film dichroic coating at an
angle of incidence that is substantially less than 45.degree..
[0017] Embodiments of the present invention provide beamsplitters
with enhanced blocking of unwanted wavelengths. For a number of
applications, such as for spectroscopic microscope applications,
this capability can reduce the performance requirements for
emission filters for isolating excitation light from the signal
path.
[0018] According to an embodiment of the present invention, there
is provided a dichroic beamsplitter comprising: [0019] a composite
prism that has at least first and second prism elements that are
coupled together along facing surfaces, wherein the respective
facing surfaces of the first and second prism elements are
equidistant from each other, [0020] wherein the composite prism has
a first flat external surface that lies within a first plane, a
second flat external surface that lies within a second plane that
is perpendicular to the first plane, a third flat external surface
that lies within a third plane that is parallel to the second
plane, and [0021] a coated surface internal to the composite prism
and comprising a multilayer thin-film dichroic beamsplitter
coating, wherein the coated surface lies within a fourth plane that
intersects at least one of the first, second, and third planes at
an angle that is less than about 25 degrees.
[0022] Additional features and advantages will be set forth in part
in the description which follows, being apparent from the
description or learned by practice of the disclosed embodiments.
The features and advantages will be realized and attained by means
of the elements and combinations particularly pointed out in the
appended claims.
[0023] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only, and are not restrictive of the scope of the
embodiments as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings.
[0025] FIG. 1 is a schematic block diagram that shows components of
a conventional fluorescence microscope.
[0026] FIG. 2 is a schematic block diagram that shows a
conventional cube beamsplitter in side view.
[0027] FIG. 3 is a schematic diagram that shows input and output
faces of a dichroic beamsplitter prism that applies for various
embodiments of the present invention.
[0028] FIG. 4A is a schematic block diagram showing a penta prism
beamsplitter for use as a dichroic beamsplitter prism according to
an embodiment of the present invention.
[0029] FIG. 4B is a schematic block diagram that shows the path of
light of a first wavelength .lamda.1 for the penta prism
beamsplitter of FIG. 4A.
[0030] FIG. 4C is a schematic block diagram that shows the path of
light of a second wavelength .lamda.2 for the penta prism
beamsplitter of FIG. 4A.
[0031] FIG. 5A is a schematic block diagram showing a modified
Pechan prism for use as a dichroic beamsplitter prism according to
an embodiment of the present invention.
[0032] FIG. 5B is an exploded view that shows component prisms that
are combined to form the modified Pechan prism of FIG. 5A.
[0033] FIG. 5C is a schematic block diagram that shows the path of
light of a first wavelength .lamda.1 for the modified Pechan prism
of FIG. 5A.
[0034] FIG. 5D is a schematic block diagram that shows the path of
light of a longer second wavelength .lamda.2 for the modified
Pechan prism of FIG. 5A.
[0035] FIG. 6 is a schematic block diagram showing a composite
wedge prism for use as a dichroic beamsplitter prism according to
an embodiment of the present invention.
[0036] FIG. 7A is a schematic block diagram of a beamsplitter with
a more complex dichroic coating for the embodiment of FIG. 6.
[0037] FIG. 7B is a graph showing the filter characteristic for the
dichroic coating used in the beamsplitter of FIG. 7A.
[0038] FIG. 8 is a schematic block diagram showing a penta prism
beamsplitter for use as a dichroic beamsplitter prism according to
an alternate embodiment of the present invention.
[0039] FIG. 9A is a schematic block diagram that shows a composite
prism beamsplitter having three internal reflections for incident
light of the first wavelength .lamda.1.
[0040] FIG. 9B is a schematic block diagram that shows the path of
light of a first wavelength .lamda.1 for the composite beamsplitter
of FIG. 9A.
[0041] FIG. 10A is a schematic block diagram that shows components
of a fluorescence microscope using a composite prism beamsplitter
according to an embodiment of the present invention.
[0042] FIG. 10B is a schematic block diagram that shows components
of a fluorescence microscope using a composite prism beamsplitter
having multilayer filter coatings on its entrance and exit surfaces
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The present description is directed in particular to
elements forming part of, or cooperating more directly with,
apparatus in accordance with the invention. It is to be understood
that elements not specifically shown or described may take various
forms well known to those skilled in the art.
[0044] Figures shown and described herein are provided in order to
illustrate key principles of operation and component relationships
along their respective optical paths according to the present
invention and are not drawn with intent to show actual size or
scale. Some exaggeration may be necessary in order to more clearly
emphasize basic structural relationships or principles of
operation. In addition, some of the figures provided may, for the
sake of clarity, show space between components that are actually in
optical contact in the claimed apparatus.
[0045] Where they are used, the terms "first", "second", "third",
and so on, do not necessarily denote any ordinal or priority
relation, but are simply used to more clearly distinguish one
element from another.
[0046] The term "oblique" is used herein to refer to an angular
relationship that is other than substantially orthogonal or
parallel, that is, at least about 5 degrees from any integer
multiple of 90 degrees.
[0047] The term "prism" or "prism element" is used herein as it is
understood in optics, to refer to a transparent optical element
that is generally in the form of an n-sided polyhedron with flat
surfaces upon which light can be incident and that is formed from a
transparent, solid material that refracts light that enters and
exits the element. It is understood that, in terms of shape and
surface outline, the optical understanding of what constitutes a
prism is less restrictive than the formal geometric definition of a
prism and encompasses that more formal definition. In optics, for
example, the term "prism" is also used in reference to a composite
element, formed from two or more component prism elements that are
glued or otherwise coupled together, including those in optical
contact, and including composite elements that are mechanically
coupled but have a thin gap at the interface between them, wherein
the gap is a fixed distance and is filled with air or epoxy, for
example.
[0048] In the context of the present disclosure, the term "penta
prism" includes prisms that, from a side view, have five sides for
light transmission or reflection, including, but not limited to,
the penta prism that has the angular arrangement used to turn an
image by 90 degrees, as used in an SLR (single-lens reflex) camera,
for example.
[0049] In the context of the present disclosure, the individual
prism elements that form a composite prism are termed "component
prisms". The descriptive term "substantially normal" means within
no more than about +/-5.0 degrees of 90 degrees, preferably as
close to 90 degrees as possible.
[0050] In the context of the present disclosure, two facing
surfaces are parallel to each other and are considered to be
equidistant from each other if the two surfaces are either in
optical contact against each other or are spaced apart by a uniform
distance that varies by no more than about +/-20 microns from an
averaged or nominal value.
[0051] In the context of the present disclosure, the terms
"configured", "treated", or "formed" are used equivalently with
respect to the fabrication of thin film filters designed to provide
a particular spectral characteristic.
[0052] Optical filters formed or configured according to
embodiments of the present invention generally employ the basic
structure of a thin film interference filter as described in the
background section. In this basic structure, a plurality of
discrete layers of material are deposited onto a surface of a
substrate in some alternating or otherwise interleaved pattern as a
filter stack, wherein the optical index between individual layers
in the filter stack changes abruptly, rather than continuously or
gradually. In conventional thin film designs, two discrete layers
are alternated, formed with thicknesses very near the
quarter-wavelength thickness of some fundamental wavelength. In
embodiments of the present invention, the same basic pattern can be
used, as well as the addition of a third or other additional
materials in the thin film stack, as needed to fine-tune filter
response.
[0053] A wide variety of materials may be used to form the
plurality of discrete material layers in the filter stack. Among
such materials, non-limiting mention is made of metals, metallic
and non-metallic oxides, transparent polymeric materials, and
so-called "soft" coatings, such as sodium aluminum fluoride
(Na.sub.3AlF.sub.6) and zinc sulfide (ZnS). Further non-limiting
mention is made of metallic oxides chosen from silicon dioxide
(SiO.sub.2), tantalum pentoxide (Ta.sub.2O.sub.5), niobium
pentoxide (Nb.sub.2O.sub.5), hafnium dioxide (HfO.sub.2), titanium
dioxide (TiO.sub.2), and aluminum pentoxide (Al.sub.2O.sub.5).
[0054] In some embodiments, the plurality of interleaved material
layers may include at least two distinct materials. As a
non-limiting example, the filters according to the present
disclosure may include a plurality of distinct alternating
Nb.sub.2O.sub.5 and SiO.sub.2 layers which have indices of
refraction of 2.3 and 1.5, respectively. Alternatively, the filters
in accordance with the present disclosure may use an interleaved
pattern with at least three distinct materials, such as distinct
Nb.sub.2O.sub.5, SiO.sub.2, and Ta.sub.2O.sub.5 layers, each layer
having a characteristic index of refraction. Of course, more than
three materials and other combinations of materials may also be
used within the interleaved layer pattern.
[0055] Generally, the filters in accordance with the present
disclosure can be manufactured using deposition methods and
techniques that are known in the art. For example, these filters
may be made with a computer controlled ion beam sputtering system,
such as is described in commonly assigned U.S. Pat. No. 7,068,430,
entitled "Method of making highly discriminating optical edge
filters and resulting products" to Clarke, et al. which is
incorporated herein by reference. In general, such a system is
capable of depositing a plurality of discrete alternating material
layers, wherein the thickness of each layer may be precisely
controlled.
[0056] Filter designs that specify the layer arrangement in
accordance with the present disclosure may be produced by known
thin-film filter design techniques. For example, these filter
designs may be produced by optimizing the filter spectra and
structure of an initial design, such as a traditional multicavity
Fabry Perot narrow bandpass interference filter, against a target
spectrum using known optical optimization routines. Non-limiting
examples of such optimization routines include the variable-metric
or simplex methods implemented in standard commercial thin-film
design software packages, such as TFCalc by Software Spectra, Inc.
of Portland, Oreg., and The Essential Macleod by Thin Film Center,
Inc., of Tucson, Ariz. A detailed description of filter design
techniques that can be used to produce filter designs according to
the present disclosure may be found in U.S. Pat. No. 7,068,430, as
noted previously.
[0057] In the context of the present disclosure, the term
"wavelength band" has its conventional meaning as understood by
those skilled in the optical arts. A wavelength band is a
continuous range of wavelengths of the electromagnetic spectrum and
may consist of one or more wavelengths.
[0058] The block diagram of FIG. 3 defines various relationships of
external and internal surfaces and light paths that are used in
common for dichroic beamsplitter prisms according to embodiments of
the present invention. A composite glass block beamsplitter 38,
formed from two or more prism elements, as described subsequently,
has a flat, externally facing surface 36a that lies in a first
plane P1. Surface 36a is perpendicular to externally facing
surfaces 36b and 36c, which are parallel to each other and that lie
in planes P2 and P3, respectively. Surface 36a may or may not have
a line of intersection with surfaces 36b or 36c that are orthogonal
to surface 36a, depending on the composite beamsplitter cube
configuration. There is a fourth surface 36d that is internal to
composite beamsplitter 38 and that has a dichroic coating. Internal
surface 36d lies within a plane P4 that intersects at least one of
planes P1, P2, and P3 at an angle .theta. that is less than about
25 degrees. In the particular example shown in the schematic of
FIG. 3, angle .theta. is at the intersection of plane P4 with plane
P3; angle .theta. is alternately at the intersection of plane P4
with P1 or P4 with P2 in alternate embodiments of the present
invention. This intersection with plane P4 is along the edge of one
of the external surfaces 36a, 36b, or 36c in the embodiments shown
herein; however, the line of intersection of plane P4 with any of
planes P1, P2, or P3 can lie outside the composite prism assembly
or within the assembly in other embodiments.
[0059] Still referring to FIG. 3, excitation light of one
wavelength band denoted by .lamda.1 is incident along an
illumination path I at or near normal incidence on surface 36a.
Internal to composite beamsplitter 38, the light at .lamda.1 at
least reflects off of an embedded multilayer thin-film dichroic
coating on surface 36d at an angle of incidence that is
substantially less than 45.degree., and that is generally less than
about 25.degree. and preferably less than or equal to about
22.5.degree.. This reflected light then exits composite
beamsplitter 38 through surface 36b at or near normal incidence and
along an emission path E, so that it is directed toward a sample
that is to be excited (downward in the representation shown in FIG.
3). The light emitted from the sample and directed back along path
E is at a different wavelength band denoted by .lamda.2. The
.lamda.2 light returns along the same optical path E in an equal
and opposite direction to that traveled by the light at .lamda.1.
The .lamda.2 light enters the composite beamsplitter through
surface 36b at or near normal incidence. Internal to glass block
composite beamsplitter 38, the light at .lamda.2 at least transmits
through the dichroic coating at an angle of incidence that is
substantially less than 45.degree., generally less than about
25.degree., and preferably less than or equal to about
22.5.degree.. This light then exits the composite beamsplitter 38
through surface 36c at or near normal incidence, along an extension
of the same optical path E through which it entered the composite
beamsplitter. Emission path E thus extends through the dichroic
beamsplitter coating that is within composite beamsplitter 38 and
has light traveling in opposite directions.
[0060] When glass block composite beamsplitter 38 of FIG. 3 is used
in the place of beamsplitter 22 in the microscopy application of
FIG. 1, for example, light for excitation from light source 12,
after reflection, is sent to the sample in one direction along path
E; light emitted from the sample travels the same path E, but in
the opposite direction, and continues through composite
beamsplitter 38 without deviation. As shown in FIG. 1, filters 20
and 32 are typically also provided when composite beamsplitter 38
is used.
[0061] Those skilled in the optical design arts can readily
recognize that light can travel in two directions along an optical
path. Thus, for example, the roles or directions of .lamda.1 light
and .lamda.2 light can be reversed from that shown in FIG. 3, with
.lamda.1 light incident on surface 36c and exiting surface 36b and
.lamda.2 light incident on surface 36b and exiting surface 36a. In
addition, the type of filter that is used for internal surface 36d
can be changed from a short wavelength pass (SWP) filter to a long
wavelength pass (LWP) filter, as needed for proper light
redirection according to its wavelength. It should also be noted
that wavelength .lamda.1 could be shorter or longer than wavelength
.lamda.2.
[0062] In detailed description or figures of subsequent
embodiments, external planes P1, P2, and P3 may not be explicitly
described or shown as they are in FIG. 3. However, it should be
noted that the described embodiments that are given subsequently
follow the planar arrangement of surfaces described with reference
to FIG. 3 for external planes P1, P2, and P3 and for internal plane
P4 that is at an angle of within about 25 degrees relative to at
least one of the external planes.
[0063] The schematic view of FIG. 4A shows a composite penta prism
beamsplitter 40 according to an embodiment of the present
invention, showing the combined paths for both wavelength .lamda.1
and .lamda.2 light. For clarity of description, FIG. 4B shows the
general path followed by light of wavelength .lamda.1, isolated
from the light of wavelength .lamda.2. FIG. 4C shows only the
wavelength .lamda.2 path, with no reflection on surfaces within
beamsplitter 40. Beamsplitter 40 is a composite prism formed from
two prism elements: a 22.5.degree. wedge prism element 48 and a
penta prism element 49. A surface 42 is one of the two
perpendicular legs of the penta prism 49, corresponding to plane P2
in FIG. 3; a surface 44 is the other perpendicular leg that accepts
incident light along illumination path I in the embodiment of the
present invention that is shown in FIG. 4A. Surface 44 corresponds
to plane P1 in FIG. 3. A dichroic coating 56 is formed along an
interface 55 that lies where a surface 46 of prism element 48 faces
a surface 54 of the penta prism element 49. Along the interface 55,
wedge prism element 48, along surface 54, is in optical contact
with penta prism element 49 along surface 46. A surface 50 is the
surface of the wedge prism element 48 that is normal to optical
path E and is parallel to surface 42 of the penta prism; surface 50
corresponds to plane P3 of FIG. 3. Angle .theta., along an edge 72
between surface 50 and surface 54 that is in optical contact with
surface 46 of the penta prism element 49, is at about 22.5 degrees.
Coating 56 corresponds to plane P4 of FIG. 3. A surface 52 opposite
to surface 46 has a highly reflective coating, such as a thin-film
coating that is highly reflective at wavelength .lamda.1. This
coating could be a metallic coating, a multilayer thin-film
dielectric coating, or a hybrid of both types. In FIG. 4A, this
coating is denoted by hatching.
[0064] FIG. 4A shows a filter characteristic curve LWP1 for
dichroic coating 56 in this embodiment, wherein wavelength .lamda.2
is longer than .lamda.1. As curve LWP1 shows, wherein T indicates
relative transmission, dichroic coating 56 is a long-wave-pass
(LWP) edge filter coating. The angle of incidence on coating 56 for
both the excitation (.lamda.1) and the emission (.lamda.2) light is
nominally about 22.5.degree.. At interface 55, coating 56 can be
formed on either surface 54 of component prism element 48 or on
surface 46 of penta prism element 49.
[0065] As can be readily appreciated from the schematic views of
FIGS. 4A through 4C, beamsplitter 40 is designed to more
effectively isolate the .lamda.1 light from the .lamda.2 light than
is done with conventional cube beamsplitters. The .lamda.1 light is
incident on dichroic coating 56 at small angles, so that coating 56
can be more efficient as an LWP filter than with conventional cube
beamsplitters.
[0066] The schematic diagram of FIG. 5A shows an alternate
embodiment of a beamsplitter 60 that employs a Pechan prism 62 with
a contacted 22.5.degree. wedge prism element 64. As shown more
clearly in the exploded view of FIG. 5B, the beamsplitter 60
arrangement makes use of three component prisms. Pechan prism 62 is
itself a composite prism formed using two component prisms 62a and
62b having an air gap 70, or gap filled with some other type of
material, between them. Prism 62b has a reflective surface 76.
Wedge prism element 64 is in optical contact with prism 62a against
a surface 74.
[0067] FIGS. 5C and 5D show the wavelength .lamda.1 and .lamda.2
light paths with the FIG. 5A arrangement. The emission light at
.lamda.2 is transmitted through the beamsplitter 60 assembly along
optical path E in the same fashion that light is normally
transmitted through a Pechan prism. Unlike other Pechan prism
embodiments, wedge prism 64 presents an input surface 66 that is
normal to light along illumination path I. The light at .lamda.1 is
transmitted through a 22.5.degree. dichroic coating 68, and then is
turned 90.degree. by reflection at the air gap 70 interface within
the Pechan prism via total internal reflection (TIR). Therefore,
for this configuration, with wavelength .lamda.2 longer than
.lamda.1, dichroic coating 68 is a short-wave-pass (SWP) edge
filter coating, as shown at a filter characteristic curve SWP1. The
light at wavelength .lamda.2 is incident on a surface 58 and is
reflected at air gap 70 by TIR. This reflected light is then
reflected from coating 68 and is directed toward a surface 78.
Following TIR at surface 78, the light is reflected from surface 76
and directed back toward air gap 70. TIR at gap 70 then redirects
the light through surface 78 along path E.
[0068] With respect to the basic schematic of FIG. 3, surface 66 of
beamsplitter 60 in FIG. 5A corresponds to plane P1; surface 58
corresponds to plane P2; surface 78 corresponds to plane P3.
Coating 68 corresponds to plane P4 and may be applied to the facing
surface of either prism element 62a or prism element 64 or with
portions applied to both facing prism element surfaces.
[0069] The schematic diagram of FIG. 6 shows an alternate
embodiment of the present invention, a composite beamsplitter prism
80 formed as a rectangular glass block comprising a pair of wedge
prism elements 80a and 80b having facing surfaces, so that each
component prism element is in optical contact with the other along
its hypotenuse. A dichroic coating 82 runs along the hypotenuse.
The example of FIG. 6 shows such a device based on 22.5.degree.
wedge prisms 80a and 80b. Angle .theta. is 22.5 degrees; paired
prism elements with angles less than 25 degrees can be used. In
this case at least some of the excitation light along illumination
path I at wavelength .lamda.1 undergoes multiple reflections from
dichroic coating 82. The excitation light at wavelength .lamda.1
then exits the composite prism assembly at or near normal incidence
to the assembly surface 92, in a direction 90.degree. relative to
the incidence direction. This light redirection can be provided by
configuring coating 82 between component prisms 80a and 80b to
reflect excitation light at .lamda.1 at a very high angle of
incidence (>>45.degree.), and reflect light at .lamda.1 at an
angle of incidence substantially smaller than 45.degree. (e.g.,
22.5.degree.), but transmit light at .lamda.2 at an angle of
incidence of 22.5.degree.. Coating 82 is thus no longer a simple
long-wave-pass or short-wave-pass edge filter coating, but rather a
coating with a more complex filter characteristic. According to an
embodiment of the present invention, the thin-film filter coating
is a multi-wavelength filter or filter that transmits multiple
wavelength bands and reflects light of wavelengths lying between
these wavelength bands. A surface 94 corresponds to plane P1 in
FIG. 3; surface 92 corresponds to plane P2; a surface 96
corresponds to plane P3.
[0070] One way to provide this more complex coating is described in
FIGS. 7A and 7B, which shows a coating at the hypotenuse of each
component prism that reflects light at .lamda..sub.1 at a very high
angle of incidence .theta.1 (>>45.degree., such as
67.5.degree. as shown), and also reflects light at .lamda..sub.1 at
an angle of incidence .theta.2 that is substantially smaller than
45.degree. (such as at 22.5.degree.). The same coating transmits
light at .lamda..sub.2 at an angle of incidence of 22.5.degree..
The bandpass filter having a characteristic curve C shown in FIG.
7B provides the more complex behavior that is needed, with the
appropriate rate of change of wavelength with angle of incidence to
satisfy the given requirements. Methods for design of a filter
having this type of characteristic are known to those skilled in
the thin film design arts.
[0071] Thus, it can be seen that the basic model described with
reference to FIG. 3 can be embodied in a number of ways, such as
those given in the examples of FIGS. 4A-7A. Other embodiments are
possible within the scope of the present invention. Advantageously,
light incidence on dichroic coated surfaces within the prism can be
much less than 45 degrees, which helps to relax requirements for
coating design and helps to improve dichroic performance over
conventional cube beamsplitter designs that use 45 degree
incidence. Total Internal Reflection (TIR) is used at air gaps and
along other surface interfaces to provide reflection with
essentially no loss of light.
[0072] It should be noted that the cross-sectional sizes of the
excitation beam and the emitted or emission beam need not be the
same. In some applications, such as with laser-excited
total-internal-reflection fluorescence (TIRF) imaging, the
excitation beam is much smaller than the emission beam and,
further, is not necessarily centered on the same axis as the
emission beam. For such systems, other embodiments are possible
within the scope of the present invention; some simplification is
possible when beam width is smaller than that shown in the FIG.
4A-7A embodiments.
[0073] In alternate embodiments, external coatings of various types
can also be used. FIG. 8 shows a similar embodiment to that shown
in FIG. 4A. A prism 90 has additional multilayer thin-film coatings
86 and 88 applied to surfaces 44 and 50 through which light at
wavelength .lamda.1 enters and at which light at wavelength
.lamda.2 exits, respectively. These coatings might be fluorescence
bandpass or edge filters, in analogy to those described in commonly
assigned U.S. Pat. No. 7,773,302 entitled "Low Cost Filter for
Fluorescence Systems" to Erdogan et al. With respect to FIG. 1,
external coatings 86 and 88 such as those shown in FIG. 8 can
supplement or substitute for filters 20 and 32. It should be noted
that external coatings 86 and 88 shown for beamsplitter 90 in FIG.
8 can be similarly applied to other beamsplitters described herein,
such as to input and output surfaces 44 and 50 of beamsplitter 40
in FIG. 4A, input and output surfaces 66 and 78 of beamsplitter 60
in FIG. 5A, and corresponding surfaces 94 and 96 of beamsplitter 80
in FIG. 6, for example.
[0074] Where external coatings are applied, as described with
reference to FIG. 8, care must be taken to make sure that light
that must be blocked by the filter coatings on the flat, externally
facing surfaces is completely accounted for. For example, in the
particular case of FIG. 8, excitation light at wavelength .lamda.1
that is not completely reflected by the dichroic coating 56 of
surface 46 will pass through the 22.5.degree. wedge prism element
48 and then impact the emission filter coating 88 on the external
exit surface of this prism. However, this light will be incident at
22.5.degree. on the emission filter coating 88, which means that
the light will see an emission filter spectrum shifted to shorter
wavelengths. If this occurs, the shifted emission passband could
potentially overlap the excitation wavelength, allowing excitation
light in the path leading to the detector. One remedy for this
problem is to design the emission filter passband to be
sufficiently red-shifted from the excitation wavelength (and/or to
ensure that an emission filter with a very low wavelength-to-angle
sensitivity is used). Another remedy is to design the dichroic
coating itself to have such high reflection at the excitation
wavelength, or range of wavelengths, that it provides sufficient
blocking by itself, without needing the excitation filter.
[0075] According to an alternate embodiment of the present
invention, the dichroic filter itself acts as the emission filter
in the system, whether a long-wave-pass type or a bandpass type.
Such high blocking using a dichroic beamsplitter is generally not
done, due to cost or difficulty. However, where there is a smaller
angle of incidence, the dichroic filter may exhibit sufficient
blocking to reduce demands on emission filter performance or may
even eliminate the need for the emission filter altogether.
[0076] In order to provide orthogonal entry and exit angles, at
least an integer number n of reflections must be provided, with
n=1, 2, 3, 4, 5, . . . Depending on the number n that is selected,
each reflection internal to the beamsplitter prism is then at 45/n
degrees incidence. By way of example, FIG. 9A shows another
embodiment of a composite beamsplitter prism 100. FIG. 9B shows
only the path of illumination light of wavelength .lamda.1; the
path for wavelength .lamda.2 light is straightforward, vertically
through prism 100 and exiting a surface 102 in the view of FIG. 9A.
The wavelength .lamda.1 light internal to the prism cube is
reflected multiple times for orthogonal exit at a surface 98.
Composite beamsplitter prism 100 has a penta prism 49 and three
wedge prisms 48a, 48b, and 48c, each in optical contact against a
corresponding surface of penta prism 49. Alternatively, prisms 48a,
48b, and 49 could comprise a single prism with the same shape as
the composite prism shown in FIG. 9A; it can be appreciated that
other shape combinations and numbers of prism elements are
possible. A coating 56 for reflecting wavelength .lamda.1 and
transmitting .lamda.2 is at the interface of wedge prism element
48c and penta prism 49. There are two reflective surfaces 52. With
respect to the planes designated in FIG. 3, input surface 104
corresponds to plane P1; surface 98 corresponds to plane P2; output
surface 102 corresponds to plane P3.
[0077] In the FIGS. 9A and 9B embodiment of the present invention,
n=3 reflections are used for wavelength .lamda.1, two from
reflective surfaces 52 and one from applied dichroic coating 56.
Each reflection is at 15 degrees (45/3=15). In general, as the
number of reflections n increases, the size of the prism also
increases.
[0078] From one aspect, the present invention provides a dichroic
beamsplitter comprising a composite prism that has at least first
and second prisms that are coupled together along facing surfaces
and are either in optical contact with each other along the facing
surfaces or spaced apart by a fixed gap such as an air gap. The
composite prism has a first flat external surface, a second flat
external surface that is perpendicular to the first flat external
surface, a third flat external surface that is parallel to the
second flat external surface, and a fourth surface internal to the
composite prism and having a multilayer thin-film dichroic
beamsplitter coating. The plane of the coated fourth surface
intersects the plane aligned with at least one of the first,
second, and third flat external surfaces at an angle that is within
25 degrees.
[0079] From another aspect of the present invention, there is
provided a beamsplitter comprising a composite prism that has at
least first and second prisms that are coupled together either in
optical contact along facing surfaces or spaced apart along the
facing surfaces by a fixed air gap. The composite prism has a first
planar external surface, a second planar external surface that is
perpendicular to the first planar external surface, a third planar
external surface that is parallel to the second planar external
surface, and a fourth planar surface within the composite prism and
having a multilayer thin-film dichroic beamsplitter coating formed
thereon, and treated to reflect and transmit light of different
wavelengths. The fourth planar surface of the composite prism
directs light of a first wavelength that is incident at a normal to
the first planar surface to be output at a normal to the second
planar surface and directs light of a second wavelength that is
incident at a normal on the second planar surface to be output at a
normal on the third planar surface. The plane of the coated fourth
surface intersects the plane of at least one of the first, second,
and third flat external surfaces at an angle that is within 25
degrees.
[0080] FIG. 10A shows a schematic diagram of a fluorescence
microscope 110 using a composite beamsplitter 38 according to an
embodiment of the present invention. Composite beamsplitter 38 can
be any of the embodiments described herein as beamsplitter 40, 60,
80, or 100, for example. In the FIG. 10A arrangement, external
filters 20 and 32 are provided to filter out the wavelength
.lamda.1 light. FIG. 10B shows an alternate arrangement, in which
any of the composite beamsplitter 38 configurations described
herein, such as beamsplitter 40, 60, 80, or 100, can be used with
applied coatings 86 and 88 in the place of external filters 20 and
32.
[0081] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention as described above, and as noted in the
appended claims, by a person of ordinary skill in the art without
departing from the scope of the invention.
* * * * *